1. Field of the Invention
The present disclosure relates to an organic electroluminescent display, and more particularly, to an organic electroluminescent display and a power supply device for the same.
2. Discussion of the Related Art
Organic electroluminescent displays (OELDs) have been proposed and developed to solve some of the problems of liquid crystal display (LCD) devices in that they are not self-luminous. The OELDs are self-luminous display devices, which emit light by electrically exciting fluorescent organic compounds. The OELDs can be driven by low voltages and can be relatively thin. OELDs that include thin film transistors as a switching element in each pixel are referred to as active matrix OELDs (AMOELDs).
FIG. 1 is a view of a pixel structure of an organic electroluminescent display according to the related art, and FIG. 1 shows a pixel including two transistors and one capacitor.
In FIG. 1, the pixel includes a switching transistor SW, a capacitor C, a driving transistor DR and an organic light-emitting diode OLED on a substrate. The switching transistor SW and the driving transistor DR are NMOS (n-channel metal-oxide-semiconductor) transistors.
A gate electrode of the switching transistor SW is connected to a scan line S, and a source electrode of the switching transistor SW is connected to a data line D. One electrode of the capacitor C is connected to a drain electrode of the switching transistor SW, and the other electrode of the capacitor C is connected to a base voltage VSS, which may be ground potential. A gate electrode of the driving transistor DR is connected to the drain electrode of the switching transistor SW and the one electrode of the capacitor C, a source electrode of the driving transistor DR is connected to the base voltage VSS, and a drain electrode of the driving transistor DR is connected to a cathode electrode of the organic light-emitting diode OLED. An anode electrode of the organic light-emitting diode OLED is connected to a power supply line VDD providing driving voltages.
A driving method of the organic electroluminescent display having the pixel structure of FIG. 1 will be explained with reference to FIG. 2. FIG. 2 shows a timing chart of the organic electroluminescent display of FIG. 1.
The switching transistor SW turns ON by a positive selection voltage Vgh, which is supplied to an nth scan line S(n) (n is a natural number) from a gate driving integrated circuit (not shown), and the capacitor C is charged due to a data voltage Vdata supplied to the data line D. The data voltage Vdata is positive because the driving transistor DR has an n-type channel. Intensity of currents flowing through the channel of the driving transistor DR depends on potential difference between the data voltage Vdata stored in the capacitor C and the driving voltage VDD, and the organic light-emitting diode OLED emits light according to the intensity of the currents.
In the organic electroluminescent display, the analog voltage applied to the driving thin film transistor DR directly affects changes in the flow of currents of the organic light-emitting diode OLED for emitting light, and this is caused by alterations of various characteristics occurring in the driving thin film transistor DR.
Recently, to solve the problem, a digital driving method has been suggested, in which the intensity of currents of the organic light-emitting diode OLED for emitting light is controlled by adjusting the driving voltage VDD provided to the driving thin film transistor DR. To perform the digital driving method, an additional unit is required to supply voltages to RGB color pixels.